I haven't figured out what the set of expressions inside the data.matrix call is supposed to do and you made no effort to explain your intent. However, your error occurs farther up the line. If you put in a debugging call to str(Symbol) you will see that Symbol will evaluate to "SPY" but that is just a character value and not an R object name. The object you wnat is named SPY and the way to retrieve an object's value when you can only have access to a character value is to use the R function get, So try adding this after the getSymbols call inside the function:

 library(quantmod) # I'm assuming this was the package in use

  ...
      Symbol=data.frame( get(Symbol) )
      str(Symbol)  # will print the result at your console
      ....
      # then perhaps you can work on what you were trying inside the data.matrix call

You will also find that the name Symbol.Adjusted will not work (since R is not a macro language). You will need to do something like:

 a=length( Symbol[[ paste0(Symbol, ".Adjusted")]] )

Oh wait. You overwrote the value for Symbol. That won't work. You need to use a different name for your dataframe. So why don't you edit your question to fix the errors I've identified so far and also describe what you are trying to do when you were using as.data.frame.